The present invention generally relates to superconducting magnetic sensors using superconducting quantum interference devices (SQUIDs), and more particularly to a superconducting magnetic sensor having a cryostat for improved sensitivity of magnetic detection.
Conventionally, various magnetometers have been employed for the measurement of weak magnetic fields. Such magnetometers include Hall-effect devices, flux gate devices, and superconducting quantum interference devices. Among others, the superconducting quantum interference magnetometers using superconducting quantum interference devices (SQUID) for the magnetic field detection are capable of measuring extremely feeble magnetic fields (less than 10.sup.-10 T) and find applications in various fields such as biomagnetic measurements, detection of gravitational radiation, and various geophysical applications.
Generally, superconducting devices have to be held in an extremely low temperature environment. For example, SQUIDs that use Josephson junctions of Nb/AlO.sub.x /Nb have to be held at a temperature below 9.2K in order to be operational. Thus, conventional superconducting magnetic sensors have been accommodated in a cryostat that typically holds liquid helium as a cooling medium. Thereby, there arises a problem in that one cannot place the superconducting pickup coil of the magnetic sensor, which is also accommodated in the cryostat, close enough to the object that is under investigation. Thereby, the conventional superconducting magnetic sensors have been vulnerable to noise and suffered from the problem of low S/N ratio. Further, in the magnetic sensors for medical applications for obtaining local distribution of magnetic field of a biological body in particular, it is desirable to provide the pickup as close as possible to the biological body under investigation.
In order to avoid the foregoing problem, there is a proposal of a cryostat shown in FIG. 1 as disclosed in the Japanese Laid-open Patent Publication 64-16976, wherein the cryostat has a double vessel structure including an inner vessel 3 for accommodating therein liquid helium 2 and an outer vessel 4 for accommodating therein liquid nitrogen 5. The inner vessel 3 is immersed in the liquid nitrogen 5 for cooling, and a SQUID magnetic sensor 1 is immersed in the liquid helium 2. Each of the vessels 3 and 4 has a double wall structure forming a vacuum jar, and the penetration of heat into the vessel 3 and the evaporation of liquid helium are successfully minimized by cooling the inner vessel 3 by the liquid nitrogen 5.
In such a cryostat structure, the superconducting magnetic sensor 1 held in the liquid helium 2 is separated from the body under investigation by the outer vessel 4 and the inner vessel 3 both having the double wall structure, as well as by the liquid nitrogen 5 and the liquid helium 2. Thus, the superconducting pickup coil of the sensor 1, if provided within the liquid helium contained in the inner vessel 3, is inevitably separated from an object 8 that is under investigation by a substantial distance. In order to avoid the problem, the apparatus of FIG. 1 employs a pickup coil 6 of a high temperature superconductor in the liquid nitrogen 5. The high temperature superconductor forming the pickup coil 6 shows the superconductivity at the liquid nitrogen temperature and is coupled magnetically to the pickup coil of the magnetic sensor 1 held within the inner vessel 2 by forming a magnetic coupling 7 across the vessel wall of the inner vessel 3. Thereby, one can reduce the distance between the pickup coil 6 and the object 8 substantially.
FIG. 2 shows an example of the superconducting magnetic sensor 1 used in the apparatus of FIG. 1.
Referring to FIG. 2, the magnetic sensor 1 is the one disclosed in the U.S. Pat. No. 5,162,731 assigned to the same assignee of the present application and includes a superconducting pickup coil 111 that establishes the magnetic coupling 7 with the pickup coil 6 of FIG. 1. The pickup coil 111 thereby establishes an interlinking with an unknown magnetic flux .PHI..sub.x via the pickup coil 6 not shown in FIG. 2 and is coupled magnetically, at a superconducting coil 112, to a superconducting detection circuit 120 that includes Josephson junctions J1 and J2, wherein the Josephson junctions J1 and J2 form a SQUID 120 together with a superconducting coil 121 that establishes a magnetic coupling M2 with the superconducting coil 112. The SQUID 120 is biased by an a.c. bias circuit 122 and produces output voltage pulses in response to each current pulse of the a.c. bias current supplied from the bias circuit by causing a transition to the finite voltage state, provided that there is an unknown magnetic flux .PHI..sub.x interlinking with the pickup coil 111.
The voltage pulses thus obtained is on the one hand supplied to an output terminal and on the other hand to a superconducting feedback unit 118 wherein the superconducting feedback unit 118 includes a superconducting write gate 135 that is coupled magnetically to a superconducting coil 133 to which the output voltage pulses are supplied from the SQUID 120. The write gate 125 includes Josephson junctions J3 and J4 as well as a superconducting coil 132 that form together a SQUID 131, wherein the SQUID 131 is supplied with the output voltage pulses via the coil 133 as bias current. Further, the coil 133 establishes a magnetic coupling with the superconducting coil 132 and causes a transition to the finite voltage state in the SQUID 131 in response to each voltage pulse. Thereby, a flux quantum enters to the SQUID in response to each voltage pulse and is stored in a superconducting coil 136 having a large inductance value.
The superconducting coil 136 in turn produces a persisting current I.sub.FB with a magnitude corresponding to the number of flux quanta stored in the superconducting coil 136, and the persisting current I.sub.FB thus produced creates a magnetic flux at a superconducting coil 117 connected to the coil 136 via a superconducting line such that the magnetic flux cancels out the unknown magnetic flux .PHI..sub.x. There, the superconducting coil 117 establishes a magnetic coupling M1 with a superconducting coil 113 forming a part of the superconducting pickup coil 111 and cancels out the induction current flowing therethrough. Until the induction current created in response to the unknown flux .PHI..sub.x is canceled out completely, the SQUID continues to produce the output voltage pulses and one can detect the direction and magnitude of the flux .PHI..sub.x by counting the number of the output voltage pulses.
Further, in order to increase the range of detection, heating elements 115 and 139 are provided for resetting the apparatus upon saturation of the superconducting coil 136 that stores the flux quanta.
As the circuit of FIG. 2 is fully described in the foregoing U.S. Pat. No. 5,162,731, further description thereof will be omitted.
Referring to FIG. 1 again, the construction of the cryostat still has a disadvantage in that the pickup coil 6 is separated from the object 8 by the inner and outer walls forming the double wall structure of the outer vessel 4 as well as the vacuum space formed therebetween. Thereby, one cannot reduce the distance between the coil 6 and the object 8 below about one centimeter. For example, assuming a thickness of about 3 mm for each of the walls forming the double wall structure of the vessel 3 and 4 and further for the vacuum space included in each double wall structure, the distance between the pickup coil 6 and the object 8 cannot be reduced below 12 mm.
Thus, the conventional apparatus of FIG. 1 has failed to provide satisfactory sensitivity for detecting the magnetic field of the object 8. When the thickness of the vacuum space is reduced for reducing the distance, on the other hand, the penetration of heat into the liquid nitrogen would become excessive because of increased radiation heat transfer. Further, such a penetration of heat tends to cause a chilling of the outer wall of the vessel 4 that in turn may injure the object 8, particularly when the object 8 is a biological body. Moreover, the apparatus of FIG. 1 has a drawback in that one cannot achieve a satisfactory magnetic coupling 7 between the pickup coil 6 and the SQUID sensor 1 across the double wall structure of the inner vessel 3, because of the large distance between the superconducting coils forming the magnetic coupling 7.